" It is also one of its best anti-counterfeit measures because it is hard to fake with wood-based paper. The particular texture of cotton paper is something that bank machines monitor. Humans are very sensitive to it, too. If there is any doubt about a banknote, there is an easy chemical test that can confirm whether it’s cotton or not. This is done in many shops using an iodine pen. When used on cellulose-based paper, the iodine reacts with the starch in the cellulose to create a pigment and so appears black. When the same pen is used on cotton paper there is no starch for the iodine to react with, and no mark appears. This basic measure allows shops to protect themselves from counterfeits produced using color photocopiers. "

― Mark Miodownik , Stuff Matters: Exploring the Marvelous Materials That Shape Our Man-Made World